1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the zoom lens. In particular, the present invention relates to a zoom lens useful as a photographic optical system of an image pickup apparatus, such as a still camera, a video camera, a monitoring camera, and the like, and to an image pickup apparatus equipped with the zoom lens.
2. Description of the Related Art
According to current market preferences, it is desired that a photographic optical system used in an image pickup apparatus that uses a solid-state image sensor has a small F-number, a wide angle of view, and a high optical performance and be a zoom lens whose total size is small. In addition, it is desired that power necessary for a drive source, which is necessary during zooming, be reduced, a minute image shake, which may occur during zooming, be suppressed, and a first lens unit be stationary during zooming so as to increase the strength of a lens barrel.
As a positive-lead type zoom lens in which a lens unit having a positive refractive power is positioned on the object side, a five-unit zoom lens, which includes first through fifth lens units having a positive refractive power, a negative refractive power, a positive refractive power, a negative refractive power, and a positive refractive power in order from the object side to the image side, has been used. U.S. Pat. No. 5,388,004 and U.S. Pat. No. 7,609,446 discuss a small-size zoom lens having a wide angle of view. More specifically, in the zoom lens discussed in U.S. Pat. No. 5,388,004 and U.S. Pat. No. 7,609,446, a first lens unit is stationary and a plurality of lens units located on the image side move during zooming and the lens units other than the first lens unit move for focusing.
If a first lens unit is set to be stationary during zooming and the angle of view is increased while maintaining and intending a predetermined F-number and a high optical performance in a positive-lead type zoom lens, the effective diameter of the first lens unit may become extremely large. Accordingly, the total size of the lens barrel may become large.
In addition, if the refractive power of each lens unit is increased to increase the angle of view in a small-size zoom lens, the amount of various aberrations may increase to too high a level to be appropriately corrected. Accordingly, it is significant that the refractive power of each lens unit and conditions for moving each lens unit during zooming are appropriately set.
In particular, unless a condition for moving an aperture stop during zooming is appropriately set, it may become difficult to achieve a zoom lens in which the effective diameter of the front lens is small and having a wide angle of view and a high optical performance for the entire zoom range.